By Eugene le Roux, FSAIRAC, and Eamonn Ryan
Would you say that heat exchangers are widely used?
Eugene le Roux. © RACA Journal
Consider this: a car can contain as many as seven heat exchangers. These include components such as the engine coolant to radiator exchanger, engine oil to engine coolant, automatic transmission fluid (ATF) to engine coolant, intercooler, air conditioning (AC) condenser, AC evaporator, and the cabin heater. In performance vehicles, you might even find an eighth exchanger — a rear axle oil-to-air cooler. Beyond the automotive world, heat exchangers are fundamental to refrigeration systems, air conditioning networks, thermal power plants, and countless industrial applications.
The simplest form of a heat exchanger
To grasp the basic concept of a heat exchanger, imagine two concentric pipes through which different fluids flow — possibly at different velocities and directions. Heat transfers from the hotter fluid to the cooler one through the pipe wall. As both fluids move, their temperatures change along the length of the exchanger. This gradual change creates a challenge: How do we calculate the effective temperature difference between the fluids?
This concept is known as temperature glide, referring to the varying temperature difference between two flowing fluids across the exchanger length.
Calculating the heat transfer
Once the LMTD is known, the heat transferred between the fluids, 𝑄, can be calculated using:
Q = htot × LMTD × A
Where:
- htot is the total heat transfer coefficient
- 𝐴 is the wet area of the inner pipe
Heat transfer from Fluid A and Fluid B
Alternatively, the same heat transfer 𝑄 can be expressed from the perspective of each fluid:
For fluid A:
Q = m˙A⋅CpA⋅ΔTA (If humanly possible, the dot is supposed to be on top of the m)
- For fluid B:
Q = m˙B⋅CpB⋅ΔTB
Where:
- m˙= mass flow rate
- Cp = specific heat capacity
- ΔT = inlet-to-outlet temperature change
Since all expressions represent the same energy transfer, they must yield the same Q, enabling engineers to back-calculate the value of htot using measurements taken during operation.
